1. Field of the Invention
The present invention relates to devices for electrically charging liquid droplets, and more particularly to devices for electrically charging selected liquid droplets in a spray utilized to coat an article with a soluble substance.
2. Description of The Prior Art
An expeditious way of providing an article with a coating of a substance is by spraying, as long as the substance is sufficiently soluble so that a liquid solution can be made for spraying purposes. However, spray processes can be inefficient when the spray particles are small and the spray process must be conducted in repeated passes or for relatively long periods of time. Under these conditions the capture rate of the spray, i.e. the amount of spray falling on the article being sprayed as opposed to the amount carried away by exhaust fumes or falling on the surrounding spray apparatus, can be 10% or less. A very low capture efficiency increases the cost of spray processes. Where expensive solutions are utilized, the expense associated with a loss of up to 90% of the solution being sprayed can be prohibitive.
Prior art solutions to this problem include one method whereby a high voltage electrical charge is placed on the spray nozzle. The article being sprayed is held at a lower electrical potential, such as ground. As the liquid droplets emerge from the spray nozzle, each droplet becomes charged with a high voltage, thereby becoming electrically attractive to the article being sprayed. This method increases capture efficiency, and therefore decreases the cost, of the spraying process.
However, some spray processes must be conducted at elevated temperatures or in a specialized environment, such as oxygen free atmosphere, etc. Economic feasibility dictates that these spray processes be conducted in spray booths which are not significantly larger than necessary to contain the spray apparatus and the article being sprayed. The charged droplets, however, are attracted to any proximate material at a lower or ground potential, not just the article desired to be coated. Since safety considerations generally preclude maintaining the entire spray booth and associated spray apparatus at the same high electrical potential as the spray nozzle, the spray booth, as well as the article being sprayed, attract the charged spray particles and become coated with the solution.
Under certain circumstances, the excess coating which lines the interior of the spray booth can be periodically washed or scraped down and recovered for reuse. However, even minor accumulation of the coating within the spray booth may cause dust and debris to fall from the surface of the spray booth onto the article being sprayed, thereby ruining the spray coating.
Moreover, if the solution being sprayed is electrically conductive, the supply lines and solution must either be at the same high potential as the high voltage spray nozzle, or an elaborate means for insulating or isolating the solution must be utilized. This can be very difficult and expensive to accomplish.
The present invention overcomes each of these problems. The liquid droplets are selectively charged, i.e. only those reaching close proximity to the article being sprayed are charged. Therefore, only the article being sprayed and the apparatus immediately adjacent to it become coated. The capture efficiency is increased, but the problems of the prior art devices are overcome.